The present invention relates to a method of forming copper complexes of azo-dyestuffs, and particularly to a method for directly producing a copperized azo-dyestuff composition of predetermined salt content.
It is well-known to produce copper complexes of ortho, ortho'-dihydroxyazo-dyestuffs by reaction of said dyestuffs in aqueous solution with cupric sulfate. This reaction is shown, for example, in U.S. Pat. No. 3,135,730. However, in many instances the resulting solution containing the copperized dyestuff, if dried directly, would yield a dyestuff composition containing more than the desired amount of inorganic salt, i.e. more salt than is utilized in the standard strength, commercial dyestuff composition. The inorganic salt is present due to the sulfate portion of the cupric sulfate which is added and due to the inorganic salts which are carried along in the preparation of the dyestuff being copperized. These latter salts are generally chlorides and/or sulfates of sodium and potassium and are introduced in a variety of ways during the dyestuff preparation, e.g. esterification with sulfuric acid and subsequent pH adjustments with sodium bicarbonate, HCl, etc.
Where the solution of copperized dyestuff contains more than the desired level of inorganic salt, as is often the case, very tedious and inefficient techniques must be employed to separate the copperized dyestuff from some of the extra salt, i.e. to increase the dyestuff/salt ratio. The technique generally employed is to precipitate out the copperized dyestuff by addition of large quantities of salt, generally 20-25% of sodium and/or potassium chloride based on the weight of liquid, filter the precipitate of copperized dyestuff containing a reduced quantity of salt, dry this product, generally by forming an aqueous slurry and sprary drying, then add salt to the final product to bring the salt level in the dyestuff composition to the standard, commercial level.
This technique has a number of disadvantages. The mother liquor from which the dyestuff is precipitated retains up to 15% of the copperized dyestuff which cannot be recovered, thus lowering the dyestuff yield. This mother liquor also presents a waste water problem since it contains both dyestuff and large quantities of salt. Large quantities of salt are consumed in the process since salt is first added to precipitate the dyestuff, then it is added again to make up the standard commercial dyestuff composition. Reaction times are extremely lengthy since salting out and filtration consume from 48 to 96 hours on a commercial run. And extra equipment, namely a filter press and a slurry vat, is required.